Petrology and depositional environments of the Hanna Formation in the Carbon Basin, Carbon County, Wyoming

The Hanna Formation of Late Paleocene - earliest Eocene age in the Carbon Basin of southcentral Wyoming consists of interbedded sandstone, conglomerate, shale, and coal. Sandstone samples from the formation were classified as feldspathic litharenite and lithic subarkose, with a few sublithic arkose. Cementing materials include calcite, silica, pyrite and limonite. Massive conglomerate and sandstone of braided alluvial plain origin alternate vertically and interfinger laterally with thick sequences of gray shale, thin fine-grained sandstone, and coal which are interpreted as meandering alluvial valley sediments. Braided alluvial plain deposits can be divided into three facies. The gravelly facies consists of coarse-grained pebbly trough-fill and poorly bedded clast supported conglomerate. The sandy facies contains fine to coarse-grained planar and trough cross-stratified sandstone, and the fine-grained facies contains minor occurrences of clay and silt associated with the braided alluvial plain deposits. Deposits of the meandering alluvial valley are dominated by thick sequences of gray shale with beds of thin, fine-grained sandstone and coal. An embedded Markov analysis of the formation shows a general fining-upward transition from braided alluvial plain deposits into meandering alluvial valley sediments. These sequences probably formed in response to gradual changes in relief between source areas and basins of deposition in between pulses of uplift in surrounding mountains.